i965: Rename "prim" parameter to "prims" where it's an array.

i965: Rename "prim" parameter to "prims" where it's an array.

Some drawing functions take a single _mesa_prim object, while others
take an array of primitives.  Both kinds of functions used a parameter
called "prim" (the singular form), which was confusing.

Using the plural form, "prims," clearly communicates that the parameter
is an array of primitives.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>